1. Field of the Invention
The present invention relates to an optical write apparatus, a method of adjusting the position of the optical write apparatus, a method of controlling the light emitting condition of the optical write apparatus, and an image forming apparatus.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 7-61035 (A) and Japanese Patent Publication No. 64-16342 (B) disclose optical write apparatuses comprising a plurality of printed boards (hereinafter referred to simply as “substrates”) each having a great number of light emitting diodes aligned in one direction. Those substrates are shifted from each other in the aligning direction of the light emitting diodes.
In the above prior art, light emitting diode array units include the substrates that are relatively small in length, having a length of A3 size paper (297×420 mm paper), for instance. The light emitting diode array units are arranged in the longitudinal direction of the light emitting diodes, thereby forming one long light emitting diode array unit. The integrated light emitting diode array units irradiate a photosensitive member with light to form a latent image. In doing so, the integrated light emitting diode array units have advantages as follows.
(a) With the above light emitting diode array units, it is possible to form a wide image, such as an A0 size image (841×1189 mm). To form such a wide image, a light emitting diode array unit using a substrate having a length of approximately 1 m can be used. However, it is costly to produce a 1 m long substrate using light emitting diodes aligned at 400 dpi (a dot pitch of 63.5 μm), because the accuracy needs to be maintained in the entire length of the light emitting diode array unit, the size of the apparatus becomes large, and the yield decreases. The integrated short light emitting diode array units disclosed in (A) and (B) can solve this problem.
(b) If one light emitting diode corresponding to 1 dot is broken in a light emitting diode array unit constituted by one substrate having a length of 1 m, the entire light emitting diode array unit needs to be replaced. In the integrated light emitting diode array units, on the other hand, only the substrate having the broken light emitting diode needs to be replaced.
Despite the above advantages, the optical write apparatus having a plurality of substrates shifted from each other in the aligning direction of the light emitting diodes has a problem in the dot switching positions at each seam between the adjacent substrates.
More specifically, as the dot pitch at 400 dpi is 63.5 μm, a dot pitch error needs to be restricted to less than 5 μm to avoid black and white lines appearing in an image.
At each seam portion, connecting the light emitting diode array units simply by connecting members, as in (A) and (B), is not sufficient to prevent the ambient temperature from varying and the light emitting diodes from generating heat. The inner temperature in an image forming apparatus employing the optical write apparatus including the light emitting diode array units rises to heat the seam portions between the substrates. As a result, the connecting members expand due to the heat, and the dot pitch at each seam portion is changed.
For example, if the distance between the seam portions of the connecting members connecting the light emitting diode array units is 20 mm, the material of the connecting members is iron having a linear expansion coefficient of 0.000012/deg, and the temperature rise is 30 degrees, a pitch shift of 7.2 μm will be caused by the temperature rise alone. Taking the accuracy in the initial positioning of the light emitting diode array units into account, the dot pitch error at the seam portions will become large enough to cause defects in an image.